__init__(self,
alphabet=None)(Constructor)

o alphabet - An optional argument specifying the alphabet
which patterns should follow. If an alphabet is set it'll be used
to verify that all patterns follow it.

Attributes:
o separator - A character to use in separating items in a signature
when it is written to a file and read back. This character should
not be in the possible alphabet of the sequences, or there will
be trouble.